This invention relates to the forming of materials by extrusion.
Our British Patent Specification No. 1,370,894 discloses and claims a process of and apparatus for the forming of materials by extrusion which can be performed continuously provided the feed material is fed in continuously. The said process in broad terms comprises the steps of feeding material into one end of a passageway formed between first and second members with the second member having a greater surface area for engaging the material to be extruded than the first member, said passageway having a blocked end remote from said one end and having at least one die orifice associated with said blocked end, and moving the passageway-defining surface of the second member relative to the passageway-defining surface of the first member in a direction towards the or each die orifice from said one end to said blocked end such that the frictional drag of the passageway-defining surface of the second member draws the material substantially in its entirety along the passageway and through the or each die orifice.
The apparatus referred to in the said British Patent Specification No. 1,370,894 in broad terms comprises movable and fixed members defining an elongate passageway therebetween, an abutment member arranged to project into and block the passageway, means defining at least one die orifice leading from the passageway and associated with the abutment member, means for continuously feeding material to be extruded into the passageway at a position spaced from the abutment member, the amount of the surface area of the passageway defined by the movable member being greater than the amount of the surface area of the passageway defined by the fixed member, whereby upon movement of the passageway-defining surface of the movable member relative to the passageway-defining surface of the fixed member, the material fed into the passageway is moved by frictional drag with the surface of the passageway in the movable member towards the abutment member and is thereby extruded substantially in its entirety through the or each die orifice.
A particular form of the said apparatus, disclosed in the said British Patent Specification No. 1,370,894, comprises a rotatable wheel member having an endless groove therein and constituting the movable member, a shoe member covering part of the length of the groove, forming a passageway therewith, and constituting the fixed member, an abutment member associated with the shoe member and projecting into the groove and blocking one end of the passageway, and at least one die orifice associated with the abutment member or shoe member. This apparatus is capable of performing continuous extrusion through the die orifice of material fed to the said passageway, provided the feed material is supplied continuously.
Described in our British Patent Specification No. 1,434,201 which is a Patent of Addition to our said British Pat. No. 1,370,894, are improvements in the apparatus specified in the preceding paragraph. They include the improvement of the shoe member having a portion projecting into and extending over a length of the passageway in front of the abutment member and of a width substantially equal to that of the passageway. Another improvement is that the wheel member with groove comprises three abutting discs, the centre disc having a diameter less than the diameter of the abutting outer discs whereby to form the groove, together with means to maintain the discs in abutting relationship. Such means may be opposed inwardly-acting compressive forces exerted on the outer faces of the outer discs.
The three part wheel is a way of avoiding stress concentration at the abrupt changes of direction manifest by the groove corner that would exist in a groove machined into a single part wheel.
During extrusion the wheel is subject to cyclic loading which is partly thermal and partly mechanical. The thermal loading derives from the temperature rise resulting from sliding friction between the feed material, the shoe member and the wheel, and from the deformation energy developed in the process. The mechanical loading derives from the pressure generated in the zones where upsetting and extrusion occur. Both of these loadings are cyclic because they occur being rotation as each part of the working surface of the wheel passes in and out of the extrusion zone. The combined cyclic loading eventually can give rise to the production of micro fatigue cracks in the groove surfaces, such cracks tending to propagate as feed material is forced into them by the said pressure until failure occurs.
In the three-part wheel there is difficulty in extracting heat from the centre part which forms the root of the groove and this component is more susceptible to failure from thermal fatigue than the side parts. The present invention seeks to provide a wheel construction which reduces or overcomes this disadvantage.